It is well known in the art to partially oxidize (“reform”) hydrocarbons such as, for example, gasoline, and to yield a gaseous mixture of fuel gases (“reformate”) including hydrogen and carbon monoxide. Such reformate, as generated, is useful as a fuel for a class of fuel cells known in the art as “solid-oxide fuel cells” (SOFC) wherein both hydrogen and carbon monoxide are oxidized by migration of an oxygen anion to produce electric power.
Another class of fuel cells, known as “proton exchange membrane” (PEM) fuel cells, is incapable of utilizing raw reformate containing carbon monoxide. Such fuel cells contain large amounts of precious metals which can become irreversibly poisoned by carbon monoxide. Thus, the higher the expected carbon monoxide level in the hydrogen stream, the greater the necessary amount of precious metal loadings in the catalyst. A motor vehicle using a fuel cell fueled with a pure hydrogen stream may need no more than, for example, $5000 worth of precious metals, whereas a motor vehicle using a fuel cell fueled with a hydrogen stream containing 20 ppm carbon monoxide may need more than $20,000 worth of precious metals.
Therefore, for PEM fuel cells, it is highly desirable to remove carbon monoxide from the hydrogen fuel feed stream, typically by preferential oxidation (PROX) catalysis to carbon dioxide, ahead of entry of the fuel stream into the fuel cell.
U.S. Pat. No. 6,162,558, issued Dec. 19, 2000, incorporated herein by reference, discloses an iridium-based PROX catalyst dispersed on and supported by a porous, inert, three-dimensional refractory carrier. Common support materials disclosed are MgO; CaO; Ca2SiO4; BaO; Ca3SiO5; ZrO2; CeO2; Cr2O3; La2O3; ThO2; alpha, delta, gamma, and theta alumina (Al2O3), and combinations thereof; silicas and silicates; sodium borosilicate; TiO2; MgAl2O4; ZnCr2O4; CaSiO3; SiO2; SiO2—Al2O3; and clay such as bentonite. The preferred carrier is a mixture of alumina and sodium borosilicate. The treatment of the feed gas by the catalyst is carried out preferably at a temperature between about 80° C. and 300° C., more preferably between about 210° C. and 260° C., resulting in carbon monoxide gas concentrations below 20 ppm, and preferably below 10 ppm. When the iridium catalyst is dispersed on a carrier comprising 30 weight percent alumina and 70 weight percent sodium borosilicate, the carbon monoxide level is reduced to as low as 4 ppm at a temperature of 220° C.
European Patent Application EP 1038832, filed Sep. 26, 1997 by Toyota Motors and published Sep. 27, 2000, discloses an apparatus that reduces the concentration of carbon monoxide in a hydrogen-rich gas, using a PROX catalyst having ruthenium as a primary component but further including, in combination with the ruthenium, another metal that extends the effective temperature range in which the selective oxidation of carbon monoxide is accelerated. The combined metal may be an alkali metal, such as lithium or potassium, or an alkaline earth metal, such as barium. Also, nickel or zinc may be used. The carrier is alumina pellets, and the feed gas treatment is carried out at a temperature between about 100° C. and 200° C. Using a catalyst containing ruthenium at a level of 0.036 mole/liter (moles Ru/volume of alumina pellets) together with potassium at a level of 0.005 mole/liter (moles K/volume of alumina pellets), the concentration of carbon monoxide is reduced to levels of 22 ppm, 4 ppm, and 8 ppm at temperatures of, respectively, 100° C., 140° C., and 200° C.
It is known in the art to use aluminum oxides or stabilized aluminum oxides in PROX catalyst compositions. Stabilized aluminates typically have stabilizer in randomized locations, not necessarily locked in the C-axis of the crystal structure. For example, barium aluminates describes many compounds such as, but not limited to BaAl2O4. The barium content of barium aluminates can range from less than about 1 wt % to more than about 60 wt %. Barium aluminates as PROX support oxides are not relatively stable to strong fluxing agents such as metal hydroxides.
It is known further in the catalytic arts to use one or more hexaaluminate compounds in the formation of catalysts, for example, for reforming hydrocarbons as fuel for gas turbines and jet engines, for partially oxidizing methane to syngas, and for combustion of gasified biomass. Known hexaaluminates typically include one or more metals in the lattice, for example, barium, manganese, lanthanum, nickel, and strontium. Hexaaluminates generally have been found to be structurally stable at higher temperatures than conventional aluminum oxides. It is not known in the art to use hexaaluminates in PROX carbon monoxide catalysis.
It is a principal object of the present invention to provide an improved oxidizing catalyst composition that is preferential for carbon monoxide and has reduced susceptibility to carbon monoxide poisoning.
It is a further object of the invention to provide such a catalyst wherein the cost of precious metal components is reduced.
It is a still further object of the invention to provide such a catalyst whereby, in a flowing mixture of gases including hydrogen and carbon monoxide, the concentration of carbon monoxide is reduced to less than 5 parts per million.